Remove getNodesInCommand debug method from Editing commands

These methods appear to be for debugging and nothing ever calls them. It's a bit
strange to want a hash set of every node all commands in the hierachy touched
as well since they're not ordered so the set doesn't really tell you what the
command did anyway.

Review URL: https://chromiumcodereview.appspot.com/23739003

git-svn-id: svn://svn.chromium.org/blink/trunk@157191 bbb929c8-8fbe-4397-9dbb-9b2b20218538